Clarified in the docs how to get to the serialized bytes.

This was a frequent source of confusion, since in all implementations
the data doesn't start at offset 0 in the buffer.

Change-Id: I045966e65928e9acd9def84e215914ecb5510653

net/FlatBuffers/FlatBufferBuilder.cs

@@ -582,6 +582,8 @@ namespace FlatBuffers
         /// </summary>
         /// <remarks>
         /// This is typically only called after you call `Finish()`.
+        /// The actual data starts at the ByteBuffer's current position,
+        /// not necessarily at `0`.
         /// </remarks>
         /// <returns>
         /// Returns the ByteBuffer for this FlatBuffer.